It is well known that fluorescent lamps have a negative incremental impedance. (W. Elenbaas, Ed., "Fluorescent Lamps and Lighting," Macmillan, London, 1959 and E. Deng and S. Cuk, "Negative Incremental Impedance and Stability of Fluorescent Lamps," IEEE Applied Power Electronics Conference, 1997 Record.) Therefore, they cannot be connected directly to an ac voltage source: they require a ballast for stable operation.
There are several requirements for fluorescent light ballasts. A particularly critical requirement for airplane applications is low radiated noise because there are strict specifications limiting radiated emissions in order to avoid risk of malfunction of airplane electronics. One of the commonly used sets of specifications is DO-160C developed by the Radio Technical Commission of Aeronautics. The specifications are given in the frequency domain and involve RF-frequency components. The range of frequencies of interest goes from 160 KHz to 1,215 MHz. Lamp voltages and currents should be sinusoidal with little distortion, so that RF-frequency harmonic components are small. Otherwise, the lamp acts like an antenna and radiated noise is high. The input voltage to the ballast can be DC or low-frequency ac voltage (50 Hz, 60 Hz or 400 Hz) and, if the lamp voltages and currents are at the same frequency of the input voltage, compliance with DO-160C is not difficult to achieve. The harmonics of the output voltage and current drop off as frequency increases, so that long before reaching 160 KHz (the minimum frequency for which there is a limit on emissions), the harmonics are negligible. Low-frequency, low-distortion lamp voltage and current are therefore desirable. Conversely, high-frequency lamp voltage and current make it harder to comply with DO-160C since the first few harmonics of lamp voltage and current fall within the frequency range of DO-160C. Other requirements are:
Lamp current stabilization--the ballast must be capable of stabilizing the lamp current. PA1 High input power factor--In the case of ac input power, high input power factor is a common requirement. PA1 Small size and weight--This requirement is particularly important for airplane applications. PA1 Lamp dimming capability--This is a common requirement. Some ballasts have a two-level dimming capability, full bright and dim. It is sometimes desirable to be able to continuously dim the lamp as a function of a given control signal. Prior-art ballasts can be divided in two categories: magnetic and electronic ballasts.
Magnetic Ballast: In the past, so-called magnetic ballasts were used extensively. Basically, a magnetic ballast consists of a large inductor (or autotransformer) placed between the ac source and the lamp. The impedance of the inductor stabilizes the lamp. The lamp voltage and current are at the same frequency of the input ac source, as indicated in FIG. 1. They are sinusoidal with very little high-frequency components, so these ballasts have low radiated noise. As a matter of fact, most ballasts used in commercial airplanes today are of this type. The input current is sinusoidal with little distortion and it has a lagging power factor due to the inductor. A capacitor at the input could be used to improve power factor. A disadvantage of this approach is large size and weight, since the line-frequency inductor used in the ballast is large. Another disadvantage is that continuous dimming is hard to implement.
High-Frequency Electronic Ballast: In the recent technical literature, there are many examples of high-frequency electronic ballasts that use switching power converters (E. Deng and S. Cuk, "Single Stage, High Power Factor, Lamp Ballast," IEEE Applied Power Electronics Conference, 1994 Record and J. M. Alonso et al., "Analysis and Experimental Results of a Single-Stage High-Power Factor Electronic Ballast Based on Flyback Converter," IEEE Applied Power Electronics Conference, 1998 Record). A block diagram of a conventional high-frequency electronic ballast is shown in FIG. 2a. A diode bridge 10 rectifies the input ac voltage, and a switching converter 11 generates a square wave voltage at the switching frequency. A matching network 12 is interposed between the switching converter output and the gas discharge lamp. This matching network is usually a high-frequency resonant filter (usually an LCC filter) tuned to a frequency equal to (or close to) the switching frequency. It attenuates all the harmonics of the square-wave voltage passing only the fundamental. Furthermore, the matching network transforms the switching converter output characteristic from a voltage source into a current source, thus ensuring stable lamp operation. A high input power factor can be obtained either by using a two-stage converter consisting of a unity-power-factor shaper followed by a high-frequency inverter or by using a single stage converter, which usually operates in discontinuous conduction mode (DCM) at the input. An example of prior-art two-stage converter is shown in FIG. 2b described in U.S. Pat. No. 5,416,387. Block 11 of FIG. 2a is implemented in the embodiment of FIG. 2b as a unity-power-factor boost converter, which ensures unity power factor, followed by a half-bridge converter comprising switches Q.sub.2 and Q.sub.3. Output capacitor C.sub.1 of the boost converter is large and stores significant energy at the input line frequency. Block 11 is followed by a matching network. An example of a prior-art single-stage converter is shown in FIG. 2calso described in the aforesaid patent. The unity-power-factor converter and the downstream dc--dc converter of FIG. 2b are combined in a single conversion stage operating in DCM of diode D.sub.1. Capacitor C.sub.1 is large and stores significant energy at the input line frequency. A matching network is used in the output to ensure stable lamp operation. Notice that in this approach of FIG. 2c the lamp current is a high frequency sinusoid at the switching frequency of the converter, as shown in FIG. 2a. As a result, there is a potential radiated noise problem.
An advantage of high-frequency ballasts is reduced size and weight of magnetic elements such as inductors and transformers due to the high-frequency operation. Another advantage is the ease of implementing continuous dimming capability by closing a current feedback control loop around the electronic ballast. In conclusion, the electronic ballast has all the desirable properties except that the lamp voltage and current are at high frequency, with a concomitant radiated noise problem.
Another type of fluorescent light ballast is described in U.S. Pat. No. 5,428,268. In this prior-art implementation the lamp voltage and current are a low frequency square wave. Notice that a square wave voltage is rich in high frequency harmonics and therefore it has significant radiated noise. The solution was also rather complicated, with a--unity power factor preregulator, a dc--dc power supply and a low frequency inverter.
Thus, most prior-art electronic ballasts for fluorescent lamps provide a sinusoidal lamp current at the switching frequency, and the high-frequency lamp voltage and current can generate significant radiated noise, which is unacceptable in noise-sensitive applications, such as fluorescent lights in airplanes.